Grip force control of predictable external loads.

The grip force used to grasp and hold an object is modulated synchronously and precisely with a self-produced load indicating predictive feedforward control. It is unclear whether an externally produced load can be anticipated with similar feedforward-timing and precision if it can be predicted, e.g., because it has a periodic time course. In the present study we tested eight healthy subjects during the compensation of an externally produced sinusoidal load with cycle duration 1.5 s and more than 700 repetitions during two successive sessions. Performance parameters characterizing the timing and precision of the grip force-load coupling were analyzed across the sessions and compared with a retention measurement on the following day and with an experimental condition when the same loads were self-produced. The time lag between the grip force and the load decreased from values greater than zero to values close to zero during the practice sessions indicating a change from a more reactive to a predictive response. In contrast, the precision and economy of the coupling showed no improvement. Performance on the second day was similar to initial performance, only some retention of feedforward timing was obvious. Precision and economy of grip force control during self-produced loading was clearly superior to external loading even after extended practice. Our findings confirm that periodic external loads are controlled by predictive feedforward mechanism after sufficient experience. However, performance was not stable and did not reach the level of self-generated loading. These results are interpreted as reflecting the significance of an efferent copy of the motor command in sensorimotor processing that may be associated with a distinct neuronal representation.